New 'directory sync' feature: mid-session attachment, reserved paths, and repository trust

What

• Directory sync now supports attaching a local machine to a session partway through, not just at the start. A new prompt explains that in this case the session's HEAD, branch, and commits stay the session's own, describes when file changes flow in each direction (on command execution and at turn boundaries), and what happens if the two checkouts diverge.
• Any path under a new reserved location is now treated as belonging to directory sync internally, and a new checkout_empty seed kind was added alongside the existing seed/clone/folder kinds. A new message warns users when commits on a branch are directory sync's own internal snapshot commits rather than real work.
• The remote-session join payload now includes a repositoryTrust field alongside directorySync.

Why These pieces lay the groundwork for syncing a local checkout with a running session safely, telling users clearly when what they're seeing is sync bookkeeping rather than their own commits, and tracking whether a repository is trusted before syncing to it.